In the lithographic printing process, a printer will often prepare a number of metal based lithographic printing plates which are used to reproduce a colored image many thousands of times. One problem in the art is that metal based presensitized printing plates are expensive and errors in the image can cause considerable unnecessary expense. It has therefore become usual to produce a relatively inexpensive color proof to assist a printer in estimating the quality of a set of transparent color separations before starting the expensive printing process. These color proofs are used to judge the color quality that will be obtained during the actual printing process. The proof must be a consistent duplicate of the desired image. Visual examination of a color proof should reveal the color rendition to be expected and any defects on the separations which might need to be altered before making the printing plates.
Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires taking al the steps necessary for actual multicolor printing. Therefore, this method of color proofing is itself costly and time consuming. Alternate color proofing methods have been developed to overcome these disadvantages. There are two general types of photoimaging methods for making color proofs; namely the overlay type and the single sheet type.
In the overlay type of color proofing, a separate transparent plastic support is used for producing each color separation film image. A number of such supports, each carrying colored images are then superimposed upon one another and held on a white sheet to produce a color proof. The overlay type of color proofing method has the disadvantage that the superimposed plastic supports tend to distort and darken the color proof, and the impression of the color proof thus prepared becomes different from copies actually obtained with a conventional printing press. Its primary advantage is that it is quickly prepared and can serve as a progressive proof by combining two or more colors in register.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images o different colors from different color separation films onto a single receptor sheet. This is done by using a single opaque support and by applying toners, photosensitive solutions or coatings of photosensitive materials of corresponding colors on the support in succession. An advantage of the single sheet type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles the actual printing process and eliminates the color distortion inherent in the overlay system.
The present invention relates to improved negative-working and positive-working proofing sheets of the single sheet, solid layer transfer class. Color proofing films of this type are composed of a sheet substrate, a colored photosensitive layer on the sheet, and an adhesive layer disposed on the photosensitive layer.
An example of a negative-working, single sheet approach is described in U.S. Pat. No. 3,671,236. The presensitized element comprises a substrate having a release surface, a pigmented photosensitive layer in contact with the release surface, a pressure sensitive adhesive layer, and a barrier layer between the photosensitive layer and the adhesive layer. The primary function of the barrier layer is to prevent pigment staining on the adhesive during development. The element is used by laminating it to a receiver via the adhesive layer. The substrate is removed, the photosensitive layer is exposed, and the material is processed in a solvent medium. These steps are generally repeated three times to produce a multicolored proof.
An example of a positive-working, single sheet approach is given in U.S. Pat. No. 4,260,673. The presensitized element comprises a substrate having a release surface, a pigmented photosensitive layer in contact with the release surface, a heat sensitive adhesive layer, and a binder layer between the pigmented layer and the adhesive layer. The binder layer comprises a photosensitive compound and prevents pigment staining of the adhesive. The element is processed like the negative-working element in U.S. Pat. No. 3,671,236.
Examples of negative-working and positive-working, single sheet approaches without preventative layers between the photosensitive and adhesive layers are described in U.S. Pat. Nos. 4,650,738 and 4,659,642, respectively. The presensitized elements comprise a sheet having a release surface, a colored photosensitive layer in contact with the release surface, and an adhesive layer directly adhered to said colored layer. The colored photosensitive layer comprises compatible resins to form a substantially uniform layer. The present invention improves upon this method by using an admixture of resins which have a certain type of solubility difference with respect to one another. The result is an intentionally formed gradient of the resin within the colored photosensitive layer. By this means, one resin preferentially migrates to the substrate while the other is conversely preferentially presented to the adhesive layer during adhesive overcoating. An advantage which is attained is that images produced with a gradient resin mixture are more stable at elevated temperatures than nongradient resins or resin mixtures.